1. Field of the Invention
The present invention relates to a radar system for detecting a target with radio waves.
2. Description of the Related Art
An FM-CW (frequency-modulation continuous wave) radar system has been developed as an in-vehicle radar system for detecting other vehicles, human beings, and obstacles.
The FM-CW radar system sends a frequency-modulated continuous wave signal as a sending signal, and mixes the sending signal with a receiver signal that is the sending signal reflected from a target to produce a beat signal for detecting a relative position and relative speed of the target from the frequency of the beat signal.
In a circuit generating the above-mentioned sending signal, a voltage controlled oscillator (referred to as a VCO below) is used. The relationship between a control voltage and an oscillatory frequency of the VCO is known in advance, so that the desired frequency modulation can be performed by controlling the control voltage of the VOC.
However, oscillatory frequency characteristics of the VCO to the control voltage is not necessarily the same with respect to each VCO. That is, the characteristics exhibit variations.
Japanese Unexamined Patent Application Publication No. 8-304532 discloses correction of triangular waves in that the period of triangular wave control voltage to a VCO is divided into (n) to correct the triangular waves by sequentially selecting two of the divided intervals. Japanese Unexamined Patent Application Publication No. 11-271428 discloses that correction data is obtained by comparing the frequency spectrum for each (n)-divided interval.
However, in any one of radar systems in the Publications mentioned above, the modulation period is divided into (n), so that if the number of divisions is increased in order to improve the correction accuracy, preparing correction data requires a long time. Also, there is a problem that the capacity of memory for storing the correction data is increased.
It is an object of the present invention to provide a radar system capable of setting a control voltage waveform of a voltage controlled oscillator for frequency-modulating a sending signal in a short time without increasing the required capacity of memory, and a characteristic adjustment method for the radar system.
A radar system according to the present invention comprises a voltage controlled oscillator for determining a sending frequency, a transmitter for sending a sending signal. The sending signal includes an ascending modulation interval, wherein a frequency gradually increases, and a descending modulation interval, wherein the frequency gradually decreases, which are alternately repeated with time in a triangular waveform by applying a voltage signal for frequency modulation to the voltage controlled oscillator. The radar system further includes a receiver for receiving a receiver signal including a reflected signal from a target, a frequency analyzer for obtaining data relating to a frequency spectrum of a beat signal that has a frequency difference between the sending signal frequency and the receiver signal frequency, whereby the radar system detects a relative distance or a relative speed of the target on the basis of the beat signal in the ascending modulation interval and the beat signal in the descending modulation interval. The radar system also includes an adjuster for adjusting time-varying characteristics of the voltage signal for frequency modulation, and a data output device for outputting the data relating to the frequency spectrum.
By this configuration, the data relating to the frequency spectrum of the beat signal is read and the time-varying characteristics of the voltage signal for frequency modulation can be adjusted based on the data relating to the frequency spectrum.
Also, according to the present invention, the frequency analyzer may obtain a peak value of a protrusion of signal intensity included in the frequency spectrum as the data relating to the frequency spectrum. Thereby, the time-varying characteristics of the voltage signal for frequency modulation can be adjusted so as to optimize the peak value of the protrusion of signal intensity generated in the frequency spectrum by the reflection signal from a target.
Also, according to the present invention, the frequency analyzer may detect a protrusion of signal intensity including in the frequency spectrum and obtain acuteness of the protrusion as the data relating to the frequency spectrum. Thereby, the time-varying characteristics of the voltage signal for frequency modulation can be adjusted so as to optimize the acuteness of the power protrusion generated in the frequency spectrum by the reflection signal from a target.
Also, according to the present invention, the frequency analyzer may obtain a plurality of differences of protrusions of signal intensity including in the frequency spectrum, each of the differences being a difference between heights of a peak and a bottom adjacent to the peak of the protrusions, as the data relating to the frequency spectrum. Thereby, the time-varying characteristics of the voltage signal for frequency modulation can be adjusted so as to optimize the resolution of a plurality of signal intensities generated in the frequency spectrum by the reflection signals from a plurality of targets located at different distances.
Also, according to the present invention, the time-varying characteristics of the voltage signal for frequency modulation may be expressed by a multinomial expression, which is at least quadric, and the time-varying characteristics of the voltage signal for frequency modulation may be configured to be adjustable by changing factors of each term. Thereby, the time-varying characteristics of the voltage signal for frequency modulation can be expressed by small number of parameters and the required memory capacity does not increase, thus enabling the entire system to be miniaturized with low cost.
Also, according to the present invention, the data relating to the frequency spectrum output from the radar system is read and the time-varying characteristics of the voltage signal for frequency modulation are adjusted so as to optimize the data. Thereby, the time-varying characteristics of the voltage signal for frequency modulation for each radar system can be optimized, and the time-varying characteristics of the voltage signal for frequency modulation for each radar system can be extremely easily adjusted.
According to the present invention, a radar system includes the adjuster means for adjusting time-varying characteristics of the voltage signal for frequency modulation and data output device for outputting the data relating to the frequency spectrum of the beat signal outside, so that the data relating to the frequency spectrum of the beat signal is read outside and time-varying characteristics of the voltage signal for frequency modulation can be adjusted based on the data relating to the frequency spectrum. Moreover, as the correction is not performed by diving the modulation period into (n) as the prior art, creation of correction data does not require a long time and memory capacity for storing the correction data does not increase. Furthermore, as a spectrum analyzer is not necessary, characteristic adjustment can be easily performed.